As the process for sealing semiconductor elements such as IC and LSI, encapsulating by molding using a molding material of an epoxy resin composition (also referred to as “an epoxy resin molding material”, hereinafter) is suitable for mass production at a low cost and has been used for a long time. With respect to the reliability, the properties of the epoxy resin compositions have been improved by improving the epoxy resin and a phenol resin used as the crosslinking agent. However, electronic instruments have decreased sizes and weights and exhibit enhanced performances as the recent trend of the market, the degree of integration of semiconductors continues to increase, and the surface mounting of semiconductor devices has been promoted. Due to the above situation, requirements to the epoxy resin compositions for encapsulating semiconductors are becoming harder, and problems which cannot be overcome with conventional epoxy resin compositions are arising.
The greatest among such problems is the phenomenon that semiconductor devices are rapidly exposed to a high temperature such as 200° C. or higher in the step of dipping into a solder or in the step of solder reflow due to adoption of the surface mounting, and cleavages caused by the stress formed by explosive vaporization of absorbed moisture take place at the inside of semiconductor devices, in particular, at the interface of the cured epoxy resin composition and various plated portions used for bonding such as gold-plated portions and silver-plated portions on semiconductor elements, lead frames and inner leads. This phenomenon causes a great damage to the reliability. Moreover, the change of solders containing lead into lead-free solders due to the environmental problems increases the temperature of the soldering treatment, and the resistance to soldering under explosive stress formed by vaporization of moisture contained in the semiconductor devices is required more greatly than before.
To overcome the decrease in the reliability due to the soldering treatment, a process in which the amount of an inorganic filler in an epoxy resin composition is increased so that a decrease in the absorption of moisture, an increase in the strength and a decrease in the expansion under heating are achieved and the resistance to soldering is improved, and a resin having a small melt viscosity is used so that low viscosity and excellent fluidity during molding is maintained, is proposed (for example Patent Reference 1). Although the resistance to soldering can be improved considerably by using this process, this process has a drawback in that fluidity is sacrificed by the increase in the amount of the inorganic filler, and cavities tend to be formed due to insufficient filling of the space in a package with the epoxy resin composition. A process in which various coupling agents such as aminosilanes and mercaptosilanes are added and fluidity and resistance to soldering are simultaneously exhibited so that formation of cleavages at the interface of plated portions and the cured epoxy resin composition is prevented, is proposed (for example, Patent Reference 2). However, this method does not succeed in obtaining an epoxy resin composition for encapsulating semiconductors exhibiting the sufficient results. Technology for satisfying the reliability without adversely affecting the fluidity and the filling property even when the amount of the inorganic filler is increased has been desired.
On the other hand, an adduct of a tertiary phosphine with a quinone exhibiting an excellent quick curing property is added to an epoxy resin composition used in the electric and electronic fields as the curing accelerator so that the curing reaction of the resin during the curing is accelerated (for example, Patent Reference 3).
When the above curing accelerator is used, the curing reaction of the epoxy resin proceeds early in the initial period of the curing reaction although the reaction is not fast since the temperature range showing the effect to accelerate curing extends to relatively low temperatures, and the molecular weight of the resin composition (the molding material) increases due to the reaction. The increase in the molecular weight increases the viscosity of the resin and, as the result, a problem such as defect molding arises due to the insufficient fluidity when a resin composition (a molding material) contains a great amount of a filler to improve the reliability.
Various attempts have been made to protect a reactive substrate using a component suppressing the curing property so that the fluidity is improved. This method is called the method of latency. For example, curing accelerators provided with latency by converting a phosphonium ion into a salt with a strongly anionic compound so that the active point of the curing accelerators is protected, are known (for example, Patent References 4 to 5). However, it is difficult that the curing property and the fluidity are simultaneously improved with these salts since the suppressing component remains throughout the initial period to the final period of the reaction. Thus, the above salts are not satisfactory.
[Patent Reference 1] Japanese Patent Application Publication No. Heisei 7 (1995)-37041 (pages 1 to 9)
[Patent Reference 2] Japanese Patent Application Laid-Open No. Heisei 8 (1996)-253555 (pages 2 to 9)
[Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 10 (1998)-25335 (page 2)
[Patent Reference 4] Japanese Patent Application Laid-Open No. 2001-98053 (page 5)
[Patent Reference 5] U.S. Pat. No. 4,171,420 (pages 2 to 4)